The present invention relates to a signal processing circuit for a telephone set.
The sound signal from a telephone transmitter and that supplied to the receiver of the same telephone set are carried over the same telephone line. Hence, the speaker's own voice or noise in the surrounding area received by the transmitter will be reproduced in the receiver. In order to prevent this phenomenon (generally referred to as sidetone), commercial telephone sets are equipped with an anti-sidetone signal processing circuit.
A block diagram of a prior art anti-sidetone signal processing circuit is shown in FIG. 1. A sound signal from a transmitter 8 is supplied to an amplifier 1 and a line driver 2, from which it is further supplied to the signal transmission line 3 of a telephone line. The output signal from the amplifier 1 is phase inverted by a negative-phase amplifier 4. The sound signal from the transmission line 3 is passed through a resistor 5 and combined with the output signal from the negative-phase amplifier 4, and the resultant signal is supplied to a receiver 7 through an amplifier 6. Since the sound signal from the transmission line 3 is combined with the inverted sound signal signal from the transmitter 8, the sound signal component from the transmitter 8 in the sound signal from the transmission line 3 is canceled and the remaining component is supplied to the receiver 7, thereby preventing sidetone.
This prior art anti-sidetone circuit is shown more specifically in FIG. 2. In the circuit shown in FIG. 2, a sound signal from a transmitter 11 is supplied to a negative-phase amplifier 14 through a resistor 12 and a capacitor 13. The output signal from the negative-phase amplifier 14 is further amplified by a VCA (voltage-controlled amplifier) 15 and supplied to an equalizer 19 composed of resistors 16 and 17 and a capacitor 18. The sound signal from the transmitter 11 is also supplied to a signal transmission line 22 via the resistor 12, capacitor 3, negative-phase amplifier 14, resistor 33 and a voltage/carrier converter circuit 21 serving as a line driver. The sound signal on the transmission line 22 is passed through an amplifier 23 and combined with the output of the equalizer 19. The composite signal at the output of the equalizer 19 is supplied to a receiver 33 via an amplifier 29 composed of an active element 24, a feedback resistor 25 and capacitors 26 and 28. The sound signal on the transmission line 22 is also passed through a VCA 30 and a resistor 31 to be combined with the sound signal from the transmitter 11. The gains of VCAs 15 and 30 are controlled by a DC detector circuit 32, which detects the DC level on the transmission line 22 and outputs a signal representing this level to the control terminals of VCAs 15 and 30. The impedance of the signal transmission line portion of the telephone line is signified by Z in FIG. 6.
In the prior art anti-sidetone signal processing circuit described above, the incoming sound signal from the transmission line 22 drives the receiver 33 after being supplied to the amplifier 29 via two routes, one including the amplifier 23, and the other composed of VCA 30, resistor 31, capacitor 13, negative-phase amplifier 14, VCA 15, and equalizer 19. The sound signal supplied from the transmitter 11 to the transmission line 22 via negative-phase amplifier 14, resistor 33 and voltage/current converter circuit 21 is shunted through the amplifier 23 and supplied to the output of the equalizer 19, where it is canceled by the signal supplied through the negative-phase amplifier 14. As a result, none of the inputs to the transmitter 11 such as the speaker's own voice and undesired noise will be reproduced in the receiver 33, thereby preventing sidetone.
The mechanism by which sidetone can be prevented by the circuit shown in FIG. 2 is hereunder described in detail. FIG. 3 is a simplified block diagram of this circuit. The following assumptions apply: VCA 30 has a gain of F; the transmitter 11 outputs a voltage of e.sub.M ; the negative-phase amplifier 14 has a gain of -A; the voltage/current converter circuit 21 has a gain of 1/R; the VCA 15 has a gain of G; the equalizer 19 has a gain of EQ; the amplifier 23 has a gain of H; and the line (transmission line) impedance is Z.
The voltage/current converter circuit 21 may be configured as shown in FIG. 4, in which an input signal is supplied to the junction point between a constant-current source 21c and a zener diode 21d via an amplifier 21a and a resistor 21b. If the resistance of the resistor 21b and the output voltage of amplifier 21a are R and e.sub.o, respectively, the current i.sub.o following through the resistor 21b is expressed as i.sub.o =e.sub.o /R, and the change in this current is produced as an output on the transmission line 22. Therefore, the voltage/current converter circuit 21 has a gain of 1/R.
The operation of the circuit having the configuration described above will be discussed below, assuming that voltage/current conversion is effected with a pure resistance R and the complex component of the equalizer 19 is used to achieve sidetone canceling.
(I) Impedance in reception mode:
The impedance Z.sub.R in the reception mode can be expressed as:
V.sub.1 (line voltage generated by the signal output from the telephone set at the other end of the line) divided by I.sub.o (line current). Therefore, EQU -I.sub.o =V.sub.1 .multidot.F.multidot.-A.multidot.1/R (1) EQU Z.sub.R =V.sub.1 /I.sub.o =R/(F.multidot.A) (2)
To attain matching between Z.sub.R and the line impedance Z, the following conditions must be satisfied: EQU Z.sub.R =Z=R/(F.multidot.A) (3) EQU R=F.multidot.A.multidot.Z (4)
In Equations (1) to (4), F denotes the gain of the VCA 30, which is controlled by the dc voltage of the line. The longer the line, the greater the line impedance Z and the lower the dc voltage of the line. Therefore, the gain F can be controlled in accordance with the length of the line so as to satisfy the condition of R=F.multidot.A.multidot.Z.
The line impedance Z is the characteristic impedance of the telephone line, which may be approximated by a complex impedance which, as shown in an equivalent circuit in FIG. 5, consists of resistors 71 and 72 and a capacitor 73.
By using a pure resistance R, reasonable matching can be attained between Z.sub.R and the complex impedance shown in FIG. 5, but it is difficult to realize complete matching. Furthermore, complex processing is necessary to execute the sidetone canceling operation to be described in (III) below.
(II) Transmission impedance and gain in transmission mode:
The impedance Z.sub.T in the transmission mode can be expressed as V.sub.1 (line voltage generated by the sound signal from the transmitter 8) divided by I.sub.o (line current). Therefore, EQU V.sub.1 ={-A.multidot.Z/(R+F.multidot.A.multidot.Z)}e.sub.M ( 5) EQU I.sub.o ={-A/(R+F.multidot.A.multidot.Z)}e.sub.M ( 6) EQU Z.sub.T =V.sub.1 /I.sub.o =Z (7)
This means that the transmission impedance is always equal to the line impedance Z in a circuit configuration that sends a signal from a constant-current source.
(III) Sidetone canceling:
The case of canceling the sidetone that occurs as a result of reproduction in the receiver 7 of the speaker's own voice received by the transmitter 8 is shown in FIG. 1. The sound signal reproduced in the receiver 7 may be determined as follows with reference to FIG. 3: ##EQU1## Since the condition for canceling the sidetone is V.sub.4 =0, Eq (4) can be rewritten as follows: EQU Z.multidot.H+R.multidot.G.multidot.EQ=0 (12) EQU Z=-EQ.multidot.R.multidot.(G/H) (13)
By satisfying Eq. (13), the sidetone is reduced to zero and eventually canceled. As in the case of gain F, the gain G of the VCA 15 is controlled according to the length of the line. The equalizer 19 has a complex component such as to cancel the complex component of line impedance Z. The actual system of the the prior art circuit shown in FIGS. 2 and 3 is designed so that the signal phase is inverted in the voice band frequency.
(IV) Reception gain:
The reception gain is given by V.sub.4 /V.sub.1. Voltages V.sub.2, V.sub.3 and V.sub.4 are expressed as follows: EQU V.sub.3 =V.sub.1 .multidot.F.multidot.-A.multidot.G.multidot.EQ (14) EQU V.sub.2 =V.sub.1 .multidot.H (15) EQU V.sub.4 =V.sub.2 +V.sub.3 =V.sub.1 (F.multidot.-A.multidot.G.multidot.EQ+H) (16)
Therefore, EQU V.sub.4 /V.sub.1 =F.multidot.-A.multidot.G.multidot.EQ+H (17)
As is clear from Eq. (17), the reception gain V.sub.4 /V.sub.1 is determined by the interrelation of all concerned parameters.
As described above, in the prior art anti-sidetone circuit, the gain F of VCA 30 is controlled to attain impedance matching, the gain G of VCA 15 is controlled to cancel sidetone, and the gain EQ of equalizer 19 is set in such a way as to cancel the complex component of the line impedance. These characteristics required for the anti-sidetone circuit have many parameters in common, so that it is difficult to design a circuit having the overall desired characteristics.